Fast Determination of Eleven Food Additives in River Water Using C18 Functionalized Magnetic Organic Polymer Nanocomposite Followed by High-Performance Liquid Chromatography

A novel magnetic nanomaterial with Fe3O4 as the core, PS-DVB as the shell layer, and the surface modified with C18 (C18−PS−DVB−Fe3O4) had been synthesized by seeded emulsion polymerization. C18−PS−DVB−Fe3O4 retains the advantages of the chemical stability, large porosity, and uniform morphology of organic polymers and has the magnetic properties of Fe3O4. A simple, flexible, and efficient magnetic dispersive solid phase extraction (Mag-dSPE) method for the extraction of preservatives, sweeteners, and colorants in river water was established. C18−PS−DVB−Fe3O4 was used as an adsorbent for Mag-dSPE and was coupled with high-performance liquid chromatography (HPLC) to detect 11 food additives: acesulfame, amaranth, benzoic acid, tartrazine, saccharin sodium, sorbic acid, dehydroacetic acid, sunset yellow, allura red, brilliant blue, and erythrosine. Under the optimum extraction conditions, combined with ChromCoreTMAQC18 (5 μm, 4.6 × 250 mm), 20 mmol/L ammonium acetate aqueous solution and methanol were used as mobile phases, and the detection wavelengths were 240 nm and 410 nm. The limits of detection (LODs) of 11 food additives were 0.6–3.1 μg/L with satisfactory recoveries ranging from 86.53% to 106.32%. And the material could be reused for five cycles without much sacrifice of extraction efficiency. The proposed method has been used to determine food additives in river water samples, and results demonstrate the applicability of the proposed C18−PS−DVB−Fe3O4 Mag-dSPE coupled with the HPLC method to environment monitoring analysis.


Introduction
With the increasing pursuit of an improved quality of life, people have much higher requirements for the quality of food.In order to improve the appearance, flavor, taste, and storage of food, food additives are usually used in the food industry [1].These food additives, according to their function, can be divided into colorants, preservatives, sweeteners, thickeners, emulsifiers, enhancers, and acidity regulators, etc. [2][3][4].The rational use of these food additives not only improves the color and taste of food, but also improves the nutritional value.However, the improper use of some food additives has led to potential threats to human health.In addition, some food additives have been shown to be potential causes of disease [5][6][7].For example, Lakshmi and co-workers found that isatins may cause permanent damage to the cornea and conjunctiva [8].Ma and co-workers reported that excessive intake of isatin has mutagenic effects and may lead to tumors.In addition, some highly toxic synthetic chemicals are also used as illegal food additives, causing serious food safety accidents [9].In addition, in the production, processing, packaging, transportation, and storage of food additives, food additives will flow into the environment due to improper operation [10].Benzoic acid, which is often used for food preservation, was detected in groundwater at concentrations of 10~27,500 µg/L [11].Acesulfame is used to improve the taste of food, and the concentration detected in surface water can be as high as 21 µg/L [12].In addition, allura red, which is used for food coloring, was detected in natural water samples at concentrations ranging from 9.9 to 77.5 µg/L [13].Therefore, the adsorption removal and enrichment detection of food additives in environmental water samples is an urgent problem to be solved.
In the production, processing, packaging, transportation, and storage of food additives, food additives will flow into the environment because of improper operation.In addition, many synthetic food additives are difficult to degrade in the body, and they are released into the environment through excretion.The ultimate destination of these food additives would be transferred to the surface water.And the decomposition of a variety of aromatic amines display carcinogenicity.Therefore, it is worth studying to establish a simple and feasible method for the detection of food additives in environmental water samples [14][15][16][17].
At present, the detection methods of food additives include liquid chromatography [18][19][20][21][22], gas chromatography [23], and colorimetric methods [24].However, the composition of the sample is very complex, and the sample needs to be pretreated before the instrument detection.Solid phase extraction (SPE) is the most widely used pretreatment technique.It includes solid phase microextraction (SPME), dispersed-solid phase extraction (d-SPE) [25], and molecularly imprinted solid phase extraction (MISPE) [26][27][28].SPE minimizes matrix effects, improves selectivity and recovery, and the detection limit of the instrument is greatly reduced [29].In 2022, a novel molecularly imprinted polymer was used as an adsorbent for MISPE to remove sodium saccharin from the sample.The detection limit was 7.0 g/L [30].SPE is more efficient and flexible than liquid-liquid extraction in terms of flexibility, simplicity, and low consumption of organic solvents.Still, the solid-liquid separation time is long after the target is adsorbed by the adsorbent.Therefore, an obvious trend in analytical chemistry is to shorten the separation time.To solve this problem, a simple and efficient magnetic dispersive solid phase extraction (Mag-dSPE) technique is developed.Mag-dSPE has the advantages of rapid solid-liquid separation, a simple method, and low cost, and it is suitable for complex matrices.The use of Mag-dSPE greatly reduces the consumption of organic solvents, which is an environmentally friendly technology [31][32][33][34].So, Mag-dSPE is also widely used in the pretreatment of food additives in recent years [35][36][37][38][39].In 2020, a novel magnetic porous nanofiber was used as an adsorbent for Mag-dSPE and HPLC to detect Sudan dye in food, with a detection limit of 0.88-1.27µg/L [40].
In this study, in order to improve the preferential adsorption ability of magnetic nanomaterials to weak polar or non-polar compounds, the materials were modified by C18.C18 modification is a commonly used functional method in reversed-phase chromatography.The working principle of reversed-phase chromatographic packing is to separate the components of a chemical mixture at an appropriate flow rate by making use of the difference in affinity between chemical substances with different chemical phases and fillers.Its separation is based on the difference between the hydrophilicity and hydrophobicity of compounds.Therefore, after the modification of C18, the specific adsorption ability of the material to non-polar compounds was increased.
At present, C18 materials are mainly silicon-based materials.Compared with organic resin, silicon-based materials have no advantage in preparation or cost.In order to prepare a kind of material which can replace silicon-based C18 material, a novel magnetic nanomaterial with Fe 3 O 4 as the core, PS-DVB as the shell layer, and the surface modified with C18 (C18−PS−DVB−Fe 3 O 4 ) was synthesized.C18−PS−DVB−Fe 3 O 4 is used as a powerful sorbent in Mag-dSPE for the separation and preconcentration of 11 food additives.In addition, an analytical procedure combining the Mag-dSPE technique with HPLC has been proposed (Figure 1).The excellent sensitivity of the developed method for 11 food additives was investigated in laboratory batch tests, and it can be applied to the routine analyses for the determination of food additives in environmental water samples.sorbent in Mag-dSPE for the separation and preconcentration of 11 food additives.In addition, an analytical procedure combining the Mag-dSPE technique with HPLC has been proposed (Figure 1).The excellent sensitivity of the developed method for 11 food additives was investigated in laboratory batch tests, and it can be applied to the routine analyses for the determination of food additives in environmental water samples.

Characterization of C18−PS−DVB−Fe3O4
In this study, a novel nanomaterial of C18−PS−DVB−Fe3O4 was synthesized, and the excellent properties of C18−PS−DVB−Fe3O4 were characterized by SEM, VSM, XPS, and BET.

Morphological Characteristics
The morphological features of Fe3O4 and C18−PS−DVB−Fe3O4 were investigated by SEM and TEM.As shown in Figure 2a, the SEM image exhibited that the particle size of Fe3O4 was uniform.The C18−PS−DVB−Fe3O4 nanoparticles had a good spherical shape, a smooth surface, and many pores.Combined with the Figures 2b,c and 3, the uniformity of the particle size needs to be improved.However, C18−PS−DVB−Fe3O4 presented an image of the core-shell structure.This was because the electron beam was difficult to pass through Fe3O4, but it can pass through organic resin.However, it had no remarkable influence on the rapid magnetic separation of the adsorbent from the sample solution.C18−PS−DVB−Fe 3 O 4 can not only be evenly dispersed in a solution without an external magnetic field, but can also be rapidly separated from the solution via a magnet within 10 s.

XPS Analysis
The chemical composition of C18−PS−DVB−Fe 3 O 4 was investigated by XPS.As shown in Figure 5a, the wide scan XPS spectra showed photoelectron lines at a binding energy of about 284.18 eV and 532.12 eV, attributed to carbon (C1s) and oxygen (O1s).The C1s high-resolution scan could be fitted into four peaks (Figure 5b) with binding energies of 283.68 eV, 284.18 eV, 286.08 eV, and 288.08 eV, which were attributed to C-N, C-H, C-O, and O-C=O, respectively.The O1s high-resolution scan could be fitted into three peaks (Figure 5c) with binding energies of 529.42 eV, 531.62 eV, and 532.57eV, which were attributed to O-H, O=C, and C-O, respectively.The successful functionalization was affirmed by the appearance of an N1s signal between 395 eV and 405 eV in a high-resolution XPS spectrum (Figure 5d).The N1s high-resolution scan could be fitted into two peaks with binding energies of 401.94 eV and 399.31 eV, which were attributed to C-N and C-N + , respectively.And no characteristic signal for iron (Fe 2p) between 710 eV and 740 eV in the high-resolution XPS spectrum was observed, confirming that there was no leakage of Fe 3 O 4 on the surface of C18−PS−DVB−Fe 3 O 4 .magnetization of C18−PS−DVB−Fe3O4 was obviously lower than that of Fe3O4, which was related to the organic polymer layer that coated the Fe3O4 surface effectively.However, it had no remarkable influence on the rapid magnetic separation of the adsorbent from the sample solution.C18−PS−DVB−Fe3O4 can not only be evenly dispersed in a solution without an external magnetic field, but can also be rapidly separated from the solution via a magnet within 10 s.

XPS Analysis
The chemical composition of C18−PS−DVB−Fe3O4 was investigated by XPS.As shown in Figure 5a, the wide scan XPS spectra showed photoelectron lines at a binding energy of about 284.18 eV and 532.12 eV, attributed to carbon (C1s) and oxygen (O1s).The C1s highresolution scan could be fitted into four peaks (Figure 5b) with binding energies of 283.68 eV, 284.18 eV, 286.08 eV, and 288.08 eV, which were attributed to C-N, C-H, C-O, and O-C=O, respectively.The O1s high-resolution scan could be fitted into three peaks (Figure 5c) with binding energies of 529.42 eV, 531.62 eV, and 532.57eV, which were attributed to O-H, O=C, and C-O, respectively.The successful functionalization was affirmed by the appearance of an N1s signal between 395 eV and 405 eV in a high-resolution XPS spectrum (Figure 5d).The N1s high-resolution scan could be fitted into two peaks with binding energies of 401.94 eV and 399.31 eV, which were attributed to C-N and C-N + , respectively.And no characteristic signal for iron (Fe 2p) between 710 eV and 740 eV in the high-resolution XPS spectrum was observed, confirming that there was no leakage of Fe3O4 on the surface of C18−PS−DVB−Fe3O4.

Specific Surface Area Analysis
To further investigate the entirely porous natures of C18−PS−DVB−Fe3O4, N2 adsorption-desorption measurements were performed (as shown in Figure S1a).The N2 adsorption isotherm accords with the type III isotherm.The test results showed that the specific surface area of this material was 372.00 m 2 /g.From the pore size distribution calculated by the desorption branch of the isotherm of the particles by the BJH method (Figure S1b), the pore size range of the material was between 0 nm and 70 nm, and the mesopore of 3.0-4.0nm was dominant, which showed that the material has a good pore structure (including micropores, mesopores, and macropores).
In addition, it was compared with the materials without C18 modification (Figure S2).The specific surface area of PS−DVB−Fe3O4 was 415.25 m 2 /g, which was lower than that

Specific Surface Area Analysis
To further investigate the entirely porous natures of C18−PS−DVB−Fe 3 O 4 , N 2 adsorption-desorption measurements were performed (as shown in Figure S1a).The N 2 adsorption isotherm accords with the type III isotherm.The test results showed that the specific surface area of this material was 372.00 m 2 /g.From the pore size distribution calculated by the desorption branch of the isotherm of the particles by the BJH method (Figure S1b), the pore size range of the material was between 0 nm and 70 nm, and the mesopore of 3.0-4.0nm was dominant, which showed that the material has a good pore structure (including micropores, mesopores, and macropores).
In addition, it was compared with the materials without C18 modification (Figure S2).The specific surface area of PS−DVB−Fe 3 O 4 was 415.25 m 2 /g, which was lower than that of C18 modified, which may be due to the blocking of some voids after C18 grafting.

FTIR Analysis
In order to further identify the functional groups on the surface of the synthesized sample, FTIR test was used, as shown in Figure 6.The tensile vibration peak at 3420 cm

Nitrogen Content Analysis
The Elementar Vario EL element analyzer was used to analyze the nitrogen content of the sample, and the results are shown in Table 1.In the synthesis process of C18 functionalized nano Fe3O4 magnetic polymer materials, GMA is the only oxygen source, and N, N-dimethyloctadecylamine was the only nitrogen source.Therefore, the grafting rate of C18 functional groups can be calculated by detecting the oxygen and nitrogen content in the materials.It could be concluded from this Table that the content of O atoms in the non-functionalized nano-magnetic polymer composite was 9.6%, that the content of GMA in the material was 0.40 g/g, and that there was 2.546 mmol of the epoxy group in the 1.0 g magnetic polymer composite.After C18 functionalization, O content decreased to 8.5%, while N content was 1.5%.It can be calculated that the contents of GMA and N, N-dimethyloctadecylamine in the 1.0 g C18 functionalized magnetic polymer composite were 0.25 g/g and 0.30 g/g, respectively.In the 1.0 g magnetic polymer composite, the C18 functional group exists at 1.766 mmol and the epoxy group exists at 1.035 mmol.It can be concluded that the grafting rate of the C18 functional group was 58.6%.

Nitrogen Content Analysis
The Elementar Vario EL element analyzer was used to analyze the nitrogen content of the sample, and the results are shown in Table 1.In the synthesis process of C18 functionalized nano Fe 3 O 4 magnetic polymer materials, GMA is the only oxygen source, and N, N-dimethyloctadecylamine was the only nitrogen source.Therefore, the grafting rate of C18 functional groups can be calculated by detecting the oxygen and nitrogen content in the materials.It could be concluded from this Table that the content of O atoms in the non-functionalized nano-magnetic polymer composite was 9.6%, that the content of GMA in the material was 0.40 g/g, and that there was 2.546 mmol of the epoxy group in the 1.0 g magnetic polymer composite.After C18 functionalization, O content decreased to 8.5%, while N content was 1.5%.It can be calculated that the contents of GMA and N, N-dimethyloctadecylamine in the 1.0 g C18 functionalized magnetic polymer composite were 0.25 g/g and 0.30 g/g, respectively.In the 1.0 g magnetic polymer composite, the C18 functional group exists at 1.766 mmol and the epoxy group exists at 1.035 mmol.It can be concluded that the grafting rate of the C18 functional group was 58.6%.In this work, C18−PS−DVB−Fe 3 O 4 nanocomposites were used as adsorbents for 11 food additives, and then the target substances were extracted from environmental water samples by the Mag-dSPE procedure.In this process, the type and concentration of the eluent, amount of adsorbent, pH of the water sample, and volume of the water sample will affect the recovery of the analyte.

Optimization of Elution Solvent
The selection of a suitable elution solvent was an important aspect of improving the extraction efficiency.The ideal elution solvent can completely elute the analyte from the surface of the extraction material.In this study, several solutions of formic acid/methanol (1.0%, v/v), ammonia/methanol (1.0%, v/v), 2.0 mmol/L ammonium acetate in methanol, 5.0 mmol/L ammonium acetate in methanol, 10.0 mmol/L ammonium acetate in methanol, 15.0 mmol/L ammonium acetate in methanol, 20.0 mmol/L ammonium acetate in methanol, and 25.0 mmol/L ammonium acetate in methanol were studied.The results are shown in Figures 7 and 8.The elution effect of the 2.0 mmol/L ammonium acetate-methanol solution was obviously better than formic acid/methanol (1.0%, v/v) and ammonia/methanol (1.0%, v/v).Using 2.0 mmol/L ammonium acetate in methanol, the recoveries of 11 food additives were 15.62-24.3%.The possible reason was that the mixture of ammonium acetate and methanol destroys the electrostatic force between C18−PS−DVB−Fe 3 O 4 and analytes.Compared with the ammonium acetate solution of 2.0 mmol/L, using the ammonium acetate solution of 20.0 mmol/L as the eluent, the elution effect of 11 kinds of food additives was the best, and the recoveries were 89.8-108.0%.It may be explained that the higher the concentration of ammonium acetate, the greater the destruction intensity of the electrostatic force between C18−PS−DVB−Fe 3 O 4 and the analyte.

Optimization of Adsorbent Dosage
The control of the adsorption dose has a great influence on the adsorption effect of the target substance.If the adsorption dose is too small, the target can not be completely adsorbed, and if the adsorption dose is too much, there will be too many irreversible points on the surface of the adsorbent so that the adsorbed target can not be eluted successfully, thus affecting the elution efficiency.In this paper, adsorbent dosages such as 10 mg, 20 mg, 30 mg, 40 mg, and 50 mg are selected for research.According to the experiment, the recoveries of 11 kinds of food additives were obtained.As shown in Figure 9, the trend of recovery was obvious when the amount of adsorbent was in the range of 10-50 mg.The adsorption effect was best when the amount of adsorbent was 40 mg, and the recoveries of 11 food additives were 88.2-105.2%.When the amount of adsorbent increased to 50 mg, the recovery did not change significantly.Therefore, the adsorbent of 40 mg is the best dosage.
tate and methanol destroys the electrostatic force between C18−PS−DVB−Fe3O4 and analytes.Compared with the ammonium acetate solution of 2.0 mmol/L, using the ammonium acetate solution of 20.0 mmol/L as the eluent, the elution effect of 11 kinds of food additives was the best, and the recoveries were 89.8-108.0%.It may be explained that the higher the concentration of ammonium acetate, the greater the destruction intensity of the electrostatic force between C18−PS−DVB−Fe3O4 and the analyte.

Optimization of Adsorbent Dosage
The control of the adsorption dose has a great influence on the adsorption effect of the target substance.If the adsorption dose is too small, the target can not be completely adsorbed, and if the adsorption dose is too much, there will be too many irreversible points on the surface of the adsorbent so that the adsorbed target can not be eluted successfully, thus affecting the elution efficiency.In this paper, adsorbent dosages such as 10

Optimization pH of Water Sample
Various pH values from 3.0 to 11.0 were used to optimize the Mag-dSPE procedure, and the results were shown in Figure 10.It indicated that, with the increasing of pH values from 3.0 to 11.0, there was no significant effect on the recovery of the target analytes, and the recoveries were in the range of 70.1-101%.With the increasing of pH values from 5.0 to 7.0, the satisfactory recoveries of the 11 food additives increased slightly.Above pH 7.0, the recoveries of 11 food additives decreased with the increase in pH within the studied pH range.Above all, 7.0 was chosen as the pH value for the investigations that followed.

Optimization pH of Water Sample
Various pH values from 3.0 to 11.0 were used to optimize the Mag-dSPE procedure, and the results were shown in Figure 10.It indicated that, with the increasing of pH values from 3.0 to 11.0, there was no significant effect on the recovery of the target analytes, and the recoveries were in the range of 70.1-101%.With the increasing of pH values from 5.0 to 7.0, the satisfactory recoveries of the 11 food additives increased slightly.Above pH 7.0, the recoveries of 11 food additives decreased with the increase in pH within the studied pH range.Above all, 7.0 was chosen as the pH value for the investigations that followed.

Optimization pH of Water Sample
Various pH values from 3.0 to 11.0 were used to optimize the Mag-dSPE procedur and the results were shown in Figure 10.It indicated that, with the increasing of pH value from 3.0 to 11.0, there was no significant effect on the recovery of the target analytes, an the recoveries were in the range of 70.1-101%.With the increasing of pH values from 5 to 7.0, the satisfactory recoveries of the 11 food additives increased slightly.Above pH 7. the recoveries of 11 food additives decreased with the increase in pH within the studie pH range.Above all, 7.0 was chosen as the pH value for the investigations that followed

Optimization of Volume of Water Sample
To obtain reliable analytical results and a high enrichment factor, obtaining satisfactory recoveries of the target substance from a large volume of sample solutions is very important.Maximum volumes ranging from 20 mL to 200 mL were studied.Following the experimental procedure, the recoveries of 11 food additives at different volumes were obtained, and the results indicate that the maximum volume was as high as 100 mL with a recovery >88.7% (Figure 11).Therefore, 100 mL of the sample solution was adopted for the preconcentration of 11 food additives from sample solutions.A high enrichment factor of 100 times was obtained because the final elute solution was 1.0 mL in these experiments.To obtain reliable analytical results and a high enrichment factor, obtaining satisfactory recoveries of the target substance from a large volume of sample solutions is very important.Maximum volumes ranging from 20 mL to 200 mL were studied.Following the experimental procedure, the recoveries of 11 food additives at different volumes were obtained, and the results indicate that the maximum volume was as high as 100 mL with a recovery >88.7% (Figure 11).Therefore, 100 mL of the sample solution was adopted for the preconcentration of 11 food additives from sample solutions.A high enrichment factor of 100 times was obtained because the final elute solution was 1.0 mL in these experiments.Finally, the optimum conditions were determined as follows: 40 mg C18−PS−DVB−Fe3O4 was used as an adsorbent to enrich 11 kinds of food additives by 100 times, then 20 mmol/L ammonium acetate solution was used as an eluent to elute the nanoparticles adsorbed with food additives, and then these were analyzed by HPLC.The obtained chromatogram is shown in Figure 12.Finally, the optimum conditions were determined as follows: 40 mg C18−PS−DVB− Fe 3 O 4 was used as an adsorbent to enrich 11 kinds of food additives by 100 times, then 20 mmol/L ammonium acetate solution was used as an eluent to elute the nanoparticles adsorbed with food additives, and then these were analyzed by HPLC.The obtained chromatogram is shown in Figure 12.

Analytical Performance of the Method
Under the magnetic solid phase extraction procedure, 11 kinds of food additives were analyzed by HPLC.It can be seen from Table 2 that 11 kinds of food additives were well enriched in the pretreatment stage.The linear range of 11 kinds of food additives was 0.1-10 µg/mL.The linear correlation coefficients were all above 0.9990.LODs and LOQs were 0.6-3.1 µg/L and 2.0-10.0µg/L, respectively.The RSD was evaluated by measuring the solution in three groups of parallel experiments.The intra-day and inter-day RSD values of 11 food additives were less than 5.9% and 8.8%, respectively.

Analytical Performance of the Method
Under the magnetic solid phase extraction procedure, 11 kinds of food additives were analyzed by HPLC.It can be seen from Table 2 that 11 kinds of food additives were well enriched in the pretreatment stage.The linear range of 11 kinds of food additives was 0.1-10 μg/mL.The linear correlation coefficients were all above 0.9990.LODs and LOQs were 0.6-3.1 μg/L and 2.0-10.0μg/L, respectively.The RSD was evaluated by measuring the solution in three groups of parallel experiments.The intra-day and inter-day RSD values of 11 food additives were less than 5.9% and 8.8%, respectively.

Regeneration and Reproducibility of C18−PS−DVB−Fe3O4
Using the same batch of C18−PS−DVB−Fe3O4 particles, the regeneration and stability of C18−PS−DVB−Fe3O4 particles as Mag-dSPE adsorbents were studied through six adsorption-desorption experiments.Before adsorption, the adsorbed particles will be washed with a desorption agent three times to remove the residual desorption agent in  Using the same batch of C18−PS−DVB−Fe 3 O 4 particles, the regeneration and stability of C18−PS−DVB−Fe 3 O 4 particles as Mag-dSPE adsorbents were studied through six adsorption-desorption experiments.Before adsorption, the adsorbed particles will be washed with a desorption agent three times to remove the residual desorption agent in the last desorption process.As shown in Figure S3, C18−PS−DVB−Fe 3 O 4 can be reused up to five times and the extraction efficiency will not be significantly reduced, but, in the sixth adsorption process, the adsorption performance will be greatly reduced.In addition, the preparation reproducibility of C18−PS−DVB−Fe 3 O 4 was evaluated, and the results showed that the RSD of the three batches of adsorbents for 11 food additives were all less than 8.8%.Therefore, C18−PS−DVB−Fe 3 O 4 had good regeneration performance and stability, and its preparation process also had good repeatability.Furthermore, the preparative reproducibility of C18−PS−DVB−Fe 3 O 4 was evaluated.The results show that the relative deviations of the recoveries for 11 food additives by using six batches of the sorbents are in the range of 3.7-9.4%.This indicates that the preparation procedure of C18−PS−DVB−Fe 3 O 4 has good repeatability and reproducibility.

Method Validation and Real Sample Analysis
To explore the practicability of the established method, it was applied to detect 11 food additives in environmental water samples.Under optimal conditions, standard solutions of food additives synthesized in 11 were added to all samples to evaluate the matrix effects.Four groups of experiments were set up; the first group was the original water sample, the standard solution addition in the second group was 1.6 µg/L, the standard solution addition in the third group was 16 µg/L, and the standard volume addition in the fourth group was 80 µg/L.The obtained results are shown in Table 3.The recoveries were 86.53-106.32%, the inter-day RSD was 0.8-8.8%,and the intra-day RSD was 0.4-5.9%.Furthermore, Benzoic acid, temptation red, and erythrin were positive in the river water samples.The detected concentrations were 4.1 µg/L, 1.2 µg/L, and 7.0 µg/L, respectively.Therefore, the detection of food additives in environmental water samples is very necessary.

Compared with Other Methods
The performance of an extraction method based on C18−PS−DVB−Fe 3 O 4 was compared with previously reported extraction methods for the determination of food additives.As can be seen in Table 4, the proposed method was demonstrated to have lower LODs values compared to other methods, and also had good RSD values and recovery.In addition, this study also compared it with PS−DVB−Fe 3 O 4 without C18 modification.The results showed that the materials without C18 modification were inferior to those without C18 modification in terms of detection limit and recovery.In the process of adsorption, PS−DVB−Fe 3 O 4 belongs to physical adsorption and had no selectivity, while C18−PS−DVB−Fe 3 O 4 belongs to chemical adsorption and had a certain selectivity, so the adsorbed material was purer, thus reducing the matrix effect and increasing the detection accuracy of the instrument.

Equipment
The magnetic and morphological characteristics of C18−PS−DVB−Fe 3 O 4 were carried out by using a Lake Shore 7404 vibrating sample magnetometer (VSM) (Westerville, OH, USA) and Zeiss Supra 55 Field emission scanning electron micrographs (SEM) (Zeiss, Oberkochen, Germany).X-ray photoelectron spectroscopy (XPS) data were obtained by Thermo Escalab 250XI (Thermo Escalab, Waltham, MA, USA).The specific surface area and pore size of the adsorbent were measured and calculated by Brunner-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods (Micromeritics, Norcross, GA, USA).Furthermore, C18−PS−DVB−Fe 3 O 4 was also analyzed by Fourier transform infrared spectroscopy (FTIR).The Fe 3 O 4 nanoparticles were prepared by the solvothermal method [47].Briefly, 2.0 g FeCl 3 •6H 2 O and 6.0 g NaAc were dissolved in 40 mL EG under ultrasonication, and then it was transferred to a Teflon-lined stainless steel autoclave (100 mL).Afterwards, the autoclave was heated to 200 • C and was maintained for 6 h, then cooled naturally to room temperature.Under the action of a magnetic field, the obtained Fe 3 O 4 particles were washed with deionized water and anhydrous ethanol and were then dried in a vacuum oven at 60 • C for 12 h.

OA Coated Fe 3 O 4 Nanoparticles
Furthermore, 1.0 g of Fe 3 O 4 as prepared above was dispersed in 200 mL of anhydrous ethanol.Then, 5.0 mL OA was slowly dripped at a reaction temperature of 80 • C and a stirring speed of 500 rpm.And then, the reaction lasted for 1.0 h.At the end of the reaction, under the action of a magnetic field, the obtained particles were washed with deionized water and anhydrous ethanol and then vacuum dried at 60 • C for 12 h.Save the brown particles and set them aside.

Synthesis of PS−DVB−Fe 3 O 4
Then, the 1.0 g Fe 3 O 4 nanoparticles coated with oleic acid, 1.0 g PVP, 100 mL 95% ethanol-water, 6.0 g ST, and 0.8 g AIBN were uniformly mixed and transferred to a 250 mL three-neck flask.The stirring speed was 150 rpm.The reaction was carried out with nitrogen for 24 h.Then, the obtained particles were washed with deionized water and anhydrous ethanol ethylene several times under the action of a magnetic field.
Afterwards, the particles were transferred to the three-neck flask of 500 mL and added to the 20 mL SDS (0.2%, m/v) solution.DBP was added to 30 mL SDS (0.2%, m/v) solution and emulsified for 1 h and then poured into the three-neck flask at a stirring speed of 150 rpm at 30 • C for 24 h.Additionally, 2.5 g PVA, 0.8 g SDS, 0.2 g BPO, 5.0 mL GMA, 7.5 mL DVB, and 14 mL toluene were added to 250 mL deionized water, emulsified for 2 h, poured into the three-neck flask, and stirred at 30 • C. The reaction was carried out with nitrogen at 70 • C for 24 h.The obtained particles were washed several times with hot water and anhydrous ethanol under the action of a magnetic field.

Synthesis of C18−PS−DVB−Fe 3 O 4
Furthermore, 25 mL methanol was used as a solvent, adding 1.0 g of PS−DVB−Fe 3 O 4 polymer composite material, and then undergoing ultrasonic dispersion for 5.0 min.Under the condition of 80 • C, condensation reflux, and 2.5 mL N, N-dimethyloctadecylamine was added to the reaction system under the stirring speed of 500 rpm for 8 h.After the reaction was completed, the C18−PS−DVB−Fe 3 O 4 particles were washed with deionized water and then washed with anhydrous ethanol several times under the action of a magnetic field.Then, it was dried in a vacuum drying box at 60 • C for 12 h.Finally, the C18−PS−DVB−Fe 3 O 4 nanoparticles were obtained.

HPLC-DAD Analysis
The chromatographic separation was performed on a ChromCore TM AQC18 (5 µm, 4.6 × 250 nm) by using 20 mmol/L ammonium acetate in water as eluent (A) and methanol as eluent (B) as the mobile phase.The detector wavelengths were 240 nm and 410 nm, and the temperature of the column thermostat was 20 • C. The injection volume was 10 µL.The separation was accomplished at a constant flow of 1.0 mL/min.The linear gradient conditions were shown in Table S1.

Figure 4
Figure 4 showed the magnetic performance of Fe 3 O 4 and C18−PS−DVB−Fe 3 O 4 , which were analyzed by VSM.The magnetic hysteresis loops reflect that the saturation value of Fe 3 O 4 was 62.67 emu/g, and C18−PS−DVB−Fe 3 O 4 was 19.93 emu/g.The saturation magnetization of C18−PS−DVB−Fe 3 O 4 was obviously lower than that of Fe 3 O 4 , which was related to the organic polymer layer that coated the Fe 3 O 4 surface effectively.However, it had no remarkable influence on the rapid magnetic separation of the adsorbent from the sample solution.C18−PS−DVB−Fe 3 O 4 can not only be evenly dispersed in a solution without an external magnetic field, but can also be rapidly separated from the solution via a magnet within 10 s.

Figure 5 .
Figure 5. X-ray photoelectron spectroscopy (XPS) analysis (a), the fit of the C1s XPS spectrum (b), the fit of the O1s XPS spectrum (c), the fit of the N1s XPS spectrum (d).

Figure 5 .
Figure 5. X-ray photoelectron spectroscopy (XPS) analysis (a), the fit of the C1s XPS spectrum (b), the fit of the O1s XPS spectrum (c), the fit of the N1s XPS spectrum (d).

Figure 7 .
Figure 7. Effect of the type of desorption solvent on the recovery (n = 6).Figure 7. Effect of the type of desorption solvent on the recovery (n = 6).

Figure 7 . 19 Figure 8 .
Figure 7. Effect of the type of desorption solvent on the recovery (n = 6).Figure 7. Effect of the type of desorption solvent on the recovery (n = 6).Molecules 2024, 29, x FOR PEER REVIEW 9 of 19

Figure 8 .
Figure 8. Influence of different concentrations of the ammonium acetate-methanol solution on the recovery (n = 6).

Table 3 .
Content, recovery rate, and relative standard deviation of 11 food additives in environmental water samples (n = 6).

Table 4 .
Comparison of the proposed method with other previously reported methods for the determination of food additives in different samples.